Engineering applications of the recently developed zinc-aluminium casting
alloys have been restricted due to certain inherent disadvantages such as
segregation. However, segregation can be overcome by thorough mh:!ng of
the melt and close temperature control or by rapid solidification of the
melt, which can be achieved by squeeze casting. A more serious problem
exists in service if components are subjected to a modest temperature
increase to about 80°C, when there is a drastic loss of strength. It was
therefore thought that the incorporation of ceramic fibres in the matrix
could improve the properties of the material at modestly elevated
temperatures.
In the majority of engineering applications, stresses exist in more than
one direction, so castings with isotropic properties are preferred and
consequently reinforcement of composite in three dimensions would be
necessary to maintain isotropic properties.
An investigation was conducted to establish the influence of squeeze
casting on the mechanical properties and structure of ZA-8, ZA-12 and ZA-27
alloys. The relationship between these factors and controlled process
variables such as die temperature and applied squeeze pressure was
established. The mechanical properties of the castings at room
temperature and the effect of ageing at 95°C on tensile strength and
dimensional changes were established. The results showed a substantial
improvement in the tensile strength of the 'as-cast' squeeze cast alloys
when compared with the 'as-cast' gravity die cast alloys. In the case of
ZA-27 alloy, squeeze casting significantly improved ductility, which is a
feature of benefit for all composite systems. The results also showed
that pressure and die temperature substantially affect dimensional changes
of the alloys when aged at 95°C.
A major aspect of the research was the evaluation of the mechanical
properties of the fibre reinforced ZA-27 alloy at elevated temperatures.
Short silicon carbide fibres were randomly oriented in the matrix to obtain
isotropic properties by a technique involving squeeze infiltration,
followed by remelting and dispersal in the melt using specially designed
equipments.
Squeeze casting was used in the final stage of the composite fabrication.
Castings of squeeze cast composite (with up to 10% volume fibre) and
squeeze infiltrated composite (with up to 18-20% volume fibre) were
produced with a sound structure and with fibres that were uniformly
distributed and randomly oriented in three dimensions.
It was found that the reaction between the fibres and molten alloy must be
closely controlled for optimum properties of the composite. In this
respect, the optimum time of contact between the fibre and the molten alloy
was experimentally determined.
It was found that the fibre supplied was of inferior tensile strength,
which resulted in poor tensile strength of the tested composite up to a
temperature of 100°C. However, the fibre brought substantial Improvement
ln the tensile strength of the composite when tested at temperatures of 150
to 250°C. The modulus of elasticity of the composite was substantially
improved at room temperature as well as at elevated temperature. The
fatigue life of the squeeze cast composite was improved compared with
squeeze cast matrix alloy (fibre-free). Squeeze cast composites with 3%
volume fibre showed an Improvement in tribological properties compared with
squeeze cast matrix alloy and squeeze cast and squeeze infiltrated
composites with higher volume percentage of fibre. Wear of cutting tools
was adversely affected by the presence of fibre.

Description:

A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.